The present invention relates to an integrated optic polarization device and method, and more particularly, to a polarization splitter and method for splitting a transverse electric (TE) mode component and a transverse magnetic (TM) mode component into two output waveguides from a polarization input, and a polarization coupler and method that couples the inputs of the TE mode component and the TM mode component together into an output optical waveguide. The present invention is implemented in integrated optics based on a single mode optical waveguide.
In integrated optics, a substrate is generally selected from various materials such as glasses, ferroelectrics, semiconductors, and polymers. LiNbO.sub.3, which is a ferroelectric, has been widely used in production of integrated optic devices since it has significant advantages over alternatives, such as low propagation losses and large electro-optic effects. LiNbO.sub.3 is an optical crystal having high birefringence, an extraordinary refractive index of 2.202, and an ordinary refractive index of 2.286 (at a wavelength of 633 nm).
Titanium indiffusion (TI) and proton exchange (PE) are representative methods for fabricating optical waveguides on LiNbO.sub.3 substrates. Titanium indiffusion refers to a method involving deposition of a thin titanium film on a portion of a surface of the LiNbO.sub.3 substrate where an optical waveguide is to be formed. The complete procedure results in formation from the thin titanium film of an optical waveguide having a thickness of about several hundred .ANG.. Thermal diffusion, performed at a high temperature of about 1,000.degree. C. for several hours, results in diffusion of the titanium into the crystal structure of the LiNbO.sub.3 substrate, which increases the refractive index of the substrate material and produces a waveguide. TI increases both the extraordinary refractive index and the ordinary refractive index, and thus a waveguide formed with TI will transmit both the TE mode component and the TM mode component of an input signal.
Proton exchange is a method for exchanging protons (H.sup.+), from a proton source such as benzoic acid, for lithium ions (Li.sup.+) in the LiNbO.sub.3 substrate. The method is implemented by depositing a mask of metal, for example, on portions of the substrate other where the optical waveguide is to be formed. The masked substrate is then dipped in the proton source at a temperature of about 200.degree. C., which causes proton exchange in the exposed (unmasked) portions of the substrate surface. PE increases the refractive index of the substrate material, which again creates a waveguide. In the case of PE, though, only the extraordinary refractive index is increased. The ordinary refractive index in fact decreases slightly, which results in a waveguide that will transmit either the TE mode component or the TM mode component, but not both.
Polarization splitters advantageously include, for example, an input waveguide and a first output waveguide, both fabricated by titanium diffusion, and a second output waveguide fabricated by proton exchange. The input waveguide receives a mixed (TE/TM) mode input signal, and the first and second output waveguides transmit respectively either TE and TM mode signals or TM and TE mode signals, depending on how the substrate is configured.
Existing polarization splitters often have a Y-type configuration in which the two output waveguides couple to the input waveguide at a branching angle. This configuration has the inherent disadvantage that highly efficient mode separation with it requires a sharp branching angle where the two output waveguides diverge from the input waveguide. But devices with this sharp branching angle, in turn, require precise determination of the distribution of the refractive index of the output waveguide for the TE mode.
An alternative approach for polarization devices entails directional coupling between proximal but separated waveguides in an integral substrate. In such an arrangement, the core portion of a first waveguide has an index of refraction higher for both modes than the index of refraction of the substrate, so that both modes propagate in the first waveguide. A second waveguide, disposed parallel to the first waveguide for a given length and separated therefrom by a small distance, has an index of refraction higher than that of the substrate for only one of the modes.
Through this arrangement, if the given length of the coupled area is chosen appropriately, the one polarized mode that will propagate in both of the waveguides is transferred preferentially from the first to the second waveguide, thereby realizing separation of the modes. This eliminates the need in Y-type splitters for a sharp branching angle to achieve complete separation of the modes. However, the two waveguides must be configured on the substrate to become separated physically. This is required to provide a sufficiently well-defined length of the coupled area and also to allow easier connection of fibers or other optical devices to the waveguides at the edges of the substrate.
Several U.S. patents have implemented this alternative approach in different forms, among them U.S. Pat. Nos. 4,669,815 to Thaniyavarn; 4,674,829 to Bulmer et al.; 4,772,084 to Bogert; 4,778,234 to Papuchon et al.; and 4,911,513 to Valet. The devices disclosed in these patents provide desirable results, but I have found that attention to the configuration of the waveguides can produce improved performance, and in particular results in an integrated optic device with low losses.